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Oxygen transport membranes (OTMs) are a promising alternative to cryogenic air separation (ASU) or pressure swing adsorption (PSA) for oxygen production. Using these ceramic membranes allows producing high purity oxygen on various scales in a continuous single-step process, at lower costs and power consumption, making it an advantageous technique for oxy-combustion in connection with carbon capture and delocalized oxygen production on a small scale. Moreover, their use in membrane reactors, directly utilizing the permeating oxygen in chemical reactions towards green chemistry, is an emerging research field. Especially dual-phase OTMs, where the membrane consists of a composite of a stable ionic conductor and a stable electronic conductor, are of high interest, because they can overcome the disadvantages of single-phase membranes like low chemical and mechanical stability at elevated temperatures and under harsh operation conditions. However, despite the progress in the development of dual-phase OTMs over the last years, and their potential applications in classic and emerging fields, challenges preventing their large-scale employment remain. This review aims to guide new studies that will promote the development and upscaling of dual-phase OTMs. Recent developments, current opportunities and challenges, and future directions of research are thoroughly discussed. Through this review paper, information about the basic working principle, properties, performance and current application in industry of dual-phase OTM membranes can be comprehended. Next to material properties, preparative methods and manufacturing are in focus, intending to accelerate development and upscaling of new materials and components. Furthermore, existing challenges and research strategies to overcome these are discussed, and focus areas and prospects of future application areas are suggested., This project was supported in part by the FLEXSNG project. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 101022432, and form the Danish Council for Independent Research (DFF) for funding within the H2Now project (Grant No. 9041-00334B). This work was also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 387282673.

Gas separation in dense ceramic membranes is driven by the partial pressure gradient across the membrane. The mixed conducting materials most commonly used are single-phase perovskites or fluorites. In recent years, the development of dual-phase systems combining a mixed ion-conducting and electron-conducting phase has increased. The advantage is that a larger number of very stable materials systems is available. The membrane designs currently used include planar, tubular, hollow-fiber, and honeycomb membranes. Each of these designs has specific advantages and disadvantages, depending on the application. Innovative joining concepts are also often needed due to the high temperatures involved. These usually involve the use of glass-ceramic sealants or reactive metal brazes. Applications focus either on the separation of gases alone, i.e., the supply of oxygen or hydrogen, or on membrane reactors. In membrane reactors, a chemical reaction occurs on one or both sides of the membrane in addition to gas separation. The supply of gases is of potential interest for power plants, for the cement, steel, and glass industries, for the medical sector, and for mobile applications. Membrane reactors can be used to produce base chemicals or synthetic fuels.

The integration of mixed ionic–electronic conducting separation membranes in catalytic membrane reactors can yield more environmentally safe and economically efficient processes. Concentration polarization effects are observed in these types of membranes when O2 permeating fluxes are significantly high. These undesired effects can be overcome by the development of new membrane reactors where mass transport and heat transfer are enhanced by adopting state-of-the-art microfabrication. In addition, careful control over the fluid dynamics regime by employing compact metallic reactors equipped with microchannels could allow the rapid extraction of the products, minimizing undesired secondary reactions. Moreover, a high membrane surface area to catalyst volume ratio can be achieved. In this work, a compact metallic reactor was developed for the integration of mixed ionic–electronic conducting ceramic membranes. An asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3–δ membrane was sealed to the metallic reactor by the reactive air brazing technique. O2 permeation was evaluated as a proof of concept, and the influence of different parameters, such as temperature, sweep gas flow rates and oxygen partial pressure in the feed gas, were evaluated.

Powders of constant morphology and quality are indispensable for reproducible ceramic manufacturing. In this study, commercially available powders were characterized regarding their microstructural properties and screened for a reproducible membrane manufacturing process, which was done by sequential tape casting. Basing on this, the slurry composition and ratio of ingredients were systematically varied in order to obtain flat, crack-free green tapes suitable for upscaling of the manufacturing process. Debinding and sintering parameters were adjusted to obtain defect-free membranes with diminished bending. The crucial parameters are the heating ramp, sintering temperature, and dwell time. The microstructure of the asymmetric membranes was investigated, leading to a support porosity of approximately 35% and a membrane layer thickness of around 20 &micro, m. Microstructure and oxygen flux are comparable to asymmetric La0.6Sr0.4Co0.2Fe0.8O3-&delta, (LSCF) membranes manufactured from custom-made powder, showing an oxygen flux of &gt, 1 mLcm&minus, 2min at 900 &deg, C in air/Ar gradient.

Considering the challenging conditions imposed by application of membranes in an asymmetric design, in particular creep resistance of the substrate material is an important parameter for the stability in long-term operation. As promising material, in terms of chemical stability, the perovskite SrTi0.75Fe0.25O3-δ has been identified in previous works. Porous supports with different microstructures have been produced using different manufacturing methods and compared to the material in its fully dense state regarding creep behaviour. The creep deformation of pressed, porous tape-cast and freeze-dried SrTi0.75Fe0.25O3-δ specimens has been investigated in the application relevant temperature range of 850–1000 °C under compressive stresses of 15, 30 and 45 MPa. A global fitting method considering all experimental data was used to derive stress exponent and activation energy of SrTi0.75Fe0.25O3-δ, which are 2.9 ± 0.4 and 402 ± 25 kJ/mol, respectively. Thus, it is suggested that the mechanism controlling creep is mainly related to dislocation climb/glide.

Perovskite-structured powders of SrTi1-xCoxO3-δ (STC-x) with nominal stoichiometry of x = 0–0.75 as well as SrTi0.75Co0.25-yFeyO3-δ (STCF-y) where y = 0–0.25 were synthesized using the Pechini method. Thermal/chemical expansion behaviour, total electrical conductivities, and oxygen permeation rates were investigated. The substitution of Ti with Co leads to an increase in both electronic and ionic conductivities and, therefore, oxygen permeability. Thermal and chemical expansions also increase slightly. The optimum Co content was found to be 25–35% due to the trade-off between phase stability and permeability. The oxygen permeation rate of STC35 is comparable to that of state-of-the-art (La,Sr)(Co,Fe)O3-δ, whereas the expansion coefficients are lower. Co-doping in STCF-y did not produce any significant differences in oxygen permeability at both high temperature and sample thickness (1.0 mm), i.e. in a solid-state diffusion-limited regime. At lower temperatures (

Hierarchically porous SrTi 0.75 Fe 0.25 O 3-δ specimens were produced through a freeze drying procedure, which yielded a channel-like porosity, reaching a value of 32%. Compressive testing was used to determine apparent elastic modulus and fracture stresses in the transverse (out-of-plane) and longitudinal (in-plane) direction, revealing a strong dependence onto pore orientation. The lower mechanical stability in the in-plane direction appears to be associated with bending mode of the pore walls, being a result of a lower resistance to crack initiation. Acoustic emissions recorded during compressive tests indicated continuous damage of the pore walls before complete failure of the specimen, which could be also confirmed by complementary in-situ compressive tests in a scanning electronic microscope.

Advanced oxygen transport membrane designs consist of a thin functional layer supported by a porous substrate material that carries mechanical loads. Creep deformation behavior is to be assessed to warrant a long-term reliable operation at elevated temperatures. Aiming towards an asymmetric composite, the current study reports and compares the creep behavior of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) perovskite porous substrate material with different porosity and pore structures in air for a temperature range of 800–1000 °C. A porosity and pore structure independent average stress exponent and activation energy are derived from the deformation data, both being representative for the LSCF material. To investigate the structural stability of the dense layer in an asymmetric membrane, sandwich samples of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) with porous substrate and dense layers on both side were tested by three-point bending with respect to creep rupture behavior of the dense layer. Creep rupture cracks were observed in the tensile surface of BSCF, but not in the case of LSCF.

The performance of ceramic gas separation membranes can be increased by reducing their thickness. To maintain the mechanical stability of the membrane, a supporting structure is needed. In this work magnesium oxide (MgO) is identified as support material for H 2 -separation membranes made of La 6-x WO 12-δ (LaWO). Therefore MgO is tested under application-related conditions in a water-gas-shift (WGS) catalytic membrane reactor for hydrogen production and shows a good stability. The sequential tape casting serves as the manufacturing method for the preparation of the asymmetric membrane structures. To show the compatibility of the two materials, detailed investigations of the sintering behaviour and stability tests are performed and show the well-matching properties of the two materials. Microstructural analyses reveal the inter-diffusion between LaWO and MgO.

Ceramic membranes can be divided into dense and porous membranes. The materials, microstructure, and manufacturing methods are described and insights into current research topics are given. By adapting the material properties and tailoring microstructures, membranes and components suitable for a variety of processes can be developed. In applications, a distinction can be made between pure gas separation and membrane reactors. In the latter ones, in addition to gas separation, a chemical reaction takes place on one or both sides of the membrane. Membrane reactors can be used to produce basic chemicals or synthetic fuels. The supply of gases can be of interest for power plants, cement, steel or glassworks as well as for the medical sector or for mobile applications.

Membrane-based oxy-combustion is a promising technology for energy efficient combustion of carbon-containing fuels with the simultaneous opportunity to capture CO2 from the resulting exhaust gas. However, oxy-combustion conditions result in special demands on the design of the ceramic membrane components due to the high pressure and temperature applied. Therefore, we have developed a planar membrane design for 4-end operation using asymmetric membranes of La0.6Sr0.4Co0.2Fe0.8O3−δ. FEM and CFD simulations have been performed in order to develop an internal channel structure that allows withstanding pressures of 5 bar on the feed side while achieving the desired O2 concentrations of 27% in the sweep gas, i.e. CO2, and an oxygen recovery rate from the feed gas of 86% at the same time. Due to the symmetric design of the membrane components, they are scalable and adaptable in size. This design has been realized in a process chain from powder to the final component consisting of thin 20 µm Membrane layer, support with 38% porosity, an inner channelled architecture and a thin (3–5 µm) porous activation layer. Particular emphasis was laid on scalable manufacturing processes in order to ensure transferability to industrial scale. The process chain is also applicable to other membrane materials suitable for any application of interest. Finally, the reproducible processing was successfully demonstrated by the fabrication of membrane components in lengths of 100 mm and widths of 70 mm.

The possibility of joining dense ceramic BCSF tubular membranes to metal alloys using a silver braze was investigated. Four different alloys (Crofer 22 APU®, Kanthal APM®, Haynes 214® and EN 1.4841) were considered and the influence of their oxide scale stability/reactivity and their thermal expansion coefficient on the stability of the final joint was evaluated. Leak tight assemblies were obtained only for steels with a thermal expansion coefficient of >16 [10−6 K−1] and protective coating. Proof-of-concept oxygen flux measurements up to 830 °C were performed on these assemblies, demonstrating the functionality of the developed hot sealing. In addition a simulation of the stresses occurring in the joint assembly during use was performed for different materials and geometries. The obtained results fit well with the experimental findings.

Aiming toward an optimization of dual phase oxygen transport membrane materials for oxygen separation applications, ceramic composites consisting of a Ce1−xGdxO2−δ (0 Ce1−xGdxO2−δ ≈ FexCo3−xO4 > CoO, and furthermore, hardness values are also in the same order. The hardness values of the obtained composites at higher impression loads reveal a stronger dependency on porosity than on composition due to similar hardness values of the main phases. Any compositional effect appears to diminish above a porosity of ≈1%.

Asymmetric membranes of mixed ionic-electronic conducting perovskite-type oxides SrTi1-xFexO3-δ (STF, x = 0.3, 0.5 and 0.7) were prepared by inverse sequential tape-casting. Both porous support (~600 μm) and functional membrane layer (~20 μm) for a given membrane assembly were made from the same composition to ensure thermochemical compatibility between the layers. Oxygen fluxes were assessed in the range 650 -1020 °C, using either (non-pressurized) ambient air or pure oxygen as feed gas at the support side of the asymmetric membrane and argon as sweep gas. Notably, similar oxygen fluxes (~1.2 × 10−6 mol cm−2 s−1) are measured through the membranes of different compositions above 950 °C when using ambient air as feed gas. This observation is interpreted to reflect the major role of the support layer resistance in rate-limiting the oxygen fluxes through the STF asymmetric membranes, which conclusion is supported by comparison of the oxygen fluxes with those measured previously through asymmetric membranes of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). A simple diffusion-convection model is used to account for the observed gas phase polarization in the porous support layers limiting the oxygen fluxes.

The overall permeation rate through asymmetric oxygen transport membranes is significantly governed by the porous support. Therefore, the microstructuring of the support's pore structure is essential to achieving the highest performances. Freeze casting is already proven to obtain hierarchical porous structures with low tortuosity, which potentially enhances the oxygen flux of oxygen transport membranes. Although a performance improvement has been reported, such improvement is not self-evident. There has yet to be a detailed comparison of the achieved microstructures in order to identify the relevant microstructural parameters. Asymmetric membranes from Ba0.5Sr0.5(Co0.8Fe0.2)0.97Zr0.03O3-δ consisting of a surface-activated 20 µm membrane layer with tape- or freeze-cast supports that have identical pore volume and layer thickness were manufactured, characterized, and compared by means of oxygen flux measurements. They were also microstructurally investigated via computed X-Ray tomography and flow simulation experiments. In the air/Ar gradient, the freeze-cast support membrane performs below the tape-cast-supported membrane. In particular, the transition zone close to the membrane, which is caused by the freezing process, significantly constrains the diffusivity and permeability of the support, and therefore leads to concentration polarizations. At temperatures below 800 °C, surface exchange kinetics at the membrane-support interface become rate-limiting.

Substituted ceria is a material which has attracted great interest in solid oxide fuel cell technology [45] . Due to its chemical stability, it may also be a promising candidate as a mixed conductive membrane for oxygen separation with flue gas contact. As part of this paper, dilatometry measurements and X-ray powder diffraction experiments were carried out on ceramic materials in air and in argon with two separate series involving the substitution of ceria; cerium was substituted with different amounts of Gd and Pr. The first substitution of Gd caused an increase of the unit cell with a small rise in linear thermal expansion in relation to pure ceria. CeO 2 − δ and Ce 0.8 Gd 0.2 O 2 − δ showed no chemical expansion in both atmospheres. The second substitution of Gd with Pr decreased the unit cell in relation to pure ceria. It is therefore understandable that Pr is preferentially introduced and shown in this work as Pr 4 + in ceria. These samples showed a remarkable chemical expansion in air and in argon. The chemical expansion displayed a clear positive correlation with increasing Pr content. The thermal expansion coefficients are comparable to the first substitution without any trend in relation to the amount of Pr. The dilatometry behaviour above 400 °C can be explained due to the release of oxygen and simultaneous reduction of Pr 4 + , forming Pr 3 + and oxygen vacancies. The results are in accordance with the two competing processes of forming vacancies (lattice contraction) and ionic radius change (lattice expansion) [16] , [27] . After cooling in air, the samples showed no residual expansion. In contrast, these samples displayed a remarkable residual expansion in argon of about 0.87% of the total relative expansion of 1.85% for Ce 0.8 Pr 0.2 O 2 − δ .

In dual-phase oxygen transport membranes we noticed that there is fast-growing interest in research for oxyfuel combustion process application. One such potential candidate is CGO-FCO (60wt% Ce0.8Gd0.2O2-δ-40wt% FeCo2O4) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO-FCO membranes of 1mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one-pot method (modified Pechini process) at 1200 degrees C for 10h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. We also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) porous layer coating over the composite. Moreover, the oxygen permeation flux of the CGO-FCO screen printed with a porous layer of 10mthick LSCF is 0.11mL/cm2 per minute at 850 degrees C with argon as sweep and air as feed gas at the rates of 50 and 250mL/min.

Advanced concepts for oxygen transport membranes focus on thin layers supported by a porous substrate. To warrant long term reliable operation of such systems, an assessment of the mechanical stability of each membrane component is required. With respect to chemical stability, cerium oxide and doped cerium oxides are promising materials for this application. The present work addresses the mechanical properties of cerium oxide variants, aiming to elucidate properties of relevance to the manufacture of reliable cerium oxide components. In particular Ce0.8Gd0.2−yPryO2−δ membrane materials and the influence of an addition 2 mol% of CoO is assessed. Materials preparation and compositional variations are compared with respect to elastic modulus, hardness and fracture toughness, as characterized using depth sensitive indentation. Furthermore, the creep behavior of doped and undoped cerium oxides, that is important for the long term structural stability at temperatures relevant to the oxygen-membranes operation, is assessed and critically discussed.

Oxygen transport membranes (OTMs) are attracting great interest for the separation of oxygen from air in an energy efficient way. In the last decade one major driver was the development of CO2 mitigation scenarios utilizing carbon capture and storage (CCS) technology for large point sources. One very promising concept is the oxyfuel technology, which realizes the combustion of fossil fuels with oxygen-enriched recirculated flue gas thus requiring enormous quantities of oxygen. OTMs can deliver this oxygen efficiently if the required heat is delivered by the process itself as it is in the case of power, cement, glass, or steel plants. Another possible application is the operation of a membrane reactor, in which oxygen is directly consumed by a chemical reaction. In this context, the research focuses on the partial oxidation of methane or even the oxidative coupling of methane to higher hydrocarbons such as ethylene, propylene, aromatics etc. Such OTMs consist of mixed ionic electronic conductors (MIEC), which can consist of a single phase MIEC material or a composite of two separate phases, each phase providing electronic or ionic conductivity, respectively. In general, the bulk transport is based on defects in the crystal lattice, i.e. oxygen vacancies and electron holes. This leads to a trade-off between permeability requiring a high defect concentration and stability, requiring a low defect concentration. Therefore, materials with limited permeability have to be used in many applications to provide a long term stable operation. In order to still reach sufficiently high permeation rates, thin supported membrane layers are developed in different shapes, mainly planar or as capillaries. In case that the membrane thickness is below a characteristic thickness, surface exchange kinetics become rate limiting because of the relatively fast diffusion through the thin bulk membrane. Consequently, the solid-gas electrochemical interfaces become more and more important, making catalytically active layers, such as porous electrodes, a key element in the future development of long term stable, high performance membranes. In this presentation the development of thin (8µm – 400µm) supported membrane layers is described using high performance perovskites, i.e. Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6-xSr0.4Co0.2Fe0.8O3-δ (LSCF). Oxygen permeation measurements with varying conditions, i.e. temperature as well as feed gas composition and sweep gas flow rates, are used to identify limiting transport processes. The overall oxygen transport process of BSCF membranes is analyzed by combining different modelling approaches. While the gas diffusion in the gas phase and the porous support is modelled by cfd and binary friction model respectively, the transport through the bulk and the surface exchange is addressed in combination using a modified Wagner approach, accounting for the electrochemically active surface areas. According to this model, either gas diffusion in the porous support or the oxygen surface exchange is limiting the transport depending on experimental conditions. Surface exchange limitations can be partly overcome using porous electrode layers made of the membrane material. Due to the high electrochemical activity of the perovskites used, an additional permeation enhancement using noble metal catalysts cannot easily be realized at high temperatures. In addition, dual phase membranes were investigated, which are much more prone to surface exchange limitations because of the limited length of the active triple phase boundaries. Electrodes made of a single phase MIEC material, i.e. LSCF, show evidence of these limitations even using 1 mm thick samples.

Advanced oxygen transport membrane designs rely upon a thin functional layer supported by a porous substrate material that carries the mechanical loads. The creep deformation behavior needs to be assessed in order to warrant a long-term reliable operation at elevated temperatures. The current study reports the creep behavior of porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ in air for a temperature range of 750–950 °C. Stress exponents and activation energies are derived from the deformation data. A comparison with the creep rates of dense material revealed a progressively increasing creep rate with increasing temperature that might be related to surface diffusional effects. Additional tests at room temperature revealed non-linear stress–strain curves and an apparent ferroelastic creep due to domain switching in the rhombohedral phase that is stable at lower temperature.

Mixed ionic electronic ceramic transport membranes have a large potential for industrial oxygen supply and carbon emission reduced fossil power plant concepts. Although permeation and phase stability are main development aspects, mechanical robustness is of concern especially for the long term performance and reliability under application relevant thermo-mechanical loads. Lanthanum strontium cobalt ferrite materials appear to be advantageous, especially with respect to CO 2 stability. However, the effect of the A-site stoichiometry on the mechanical properties needs to be assessed. Furthermore, advanced design concepts rely on thin layers supported by a porous substrate and therefore also the porosity is an important factor. Hence, these aspects were investigated for dense and porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ and dense La 0.38 Sr 0.62 Co 0.2 Fe 0.8 O 3− δ . The specimens were investigated using a ring-on-ring bending set-up. The work summarizes the effect of the stoichiometry and porosity on the mechanical properties and compares the temperature dependencies of elastic moduli and fracture stresses.

Doped ceria pellets with the composition Ce0.8Gd0.2-xPrxO2-δ with x = 0.15, 0.1, 0.05, and 0.03 were investigated with a special focus on the partial conductivities in the temperature range of 200–600°C. Temperature dependent conductivity provided by impedance spectroscopy in air was compared to measurements of the oxygen partial pressure dependent electronic conductivity. The electronic conductivity was analyzed down to 200°C by using a modified Hebb-Wagner setup with encapsulated Pt microcontacts. A small polaron hopping process introduced by reduction of praseodymium was found to have a pronounced influence on the electronic conductivity at low temperatures. A splitting of the maximum introduced by praseodymium small polaron hopping was observed. Especially for the compositions x ≤ 0.1, a strong deviation of the electronic conductivity curves from the standard acceptor doping case was measured due to superimposed electronic conductivity from the Pr3+/4+ redox reaction.

New oxygen transport membrane materials based on SrTi1-xFexO3-δ, synthesized through solid state reaction and processed via tape casting were characterized with respect to their mechanical behaviour via depth-sensitive indentation and ring-on-ring flexural testing. The elastic moduli obtained by indentation with 1 N load for SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5) specimens were 147 ± 10 GPa, 123 ± 6 GPa and 158 ± 10 GPa, respectively. Fracture stress was accessed by ring-on-ring testing performed at 100 N/min and the obtained results were 92 ± 9 MPa, 117 ± 15 MPa, and 100 ± 15 MPa for SrTi0.75Fe0.25O3, SrTi0.65Fe0.35O3, and SrTi0.5Fe0.5O3 respectively. Ring-on-ring tests conducted at different loading rates gave access to subcritical crack growth sensitivity and aided the prediction of the materials’ lifetime through stress-time-probability diagrams, where SrTi1-xFexO3-δ (x = 0.25, 0.35, 0.5) may resist for 1 year with a failure probability of 0.1% at least 15 MPa, 22 MPa, and 12 MPa respectively.

Ceramic oxide membranes are widely being researched for Carbon Capture and Storage/Utilization sector applications. Foreseen applications of these membranes are oxygen generation for oxyfuel combustion in e.g. power plants, glass-, cement- or steel production. Major drawback with Mixed Ionic and Electronic Conducting (MIEC) perovskite structure membranes is their limited long term stability at high temperatures in aggressive atmospheres. Dual phase composite membranes have been reported to excel overcoming this drawback. In addition to performance evaluation, Ce0.8Gd0.2O2-δ – FeCo2O4 (CGO-FCO) membranes were subjected to stability test in flue gas conditions closely mimicking industrial flue gas atmosphere. The dual phase composites are investigated for their phase stability at the operating temperature of 850 °C in a gradient of oxygen chemical potential. The composites were also exposed to a series of gas mixtures over a period of time at their operating temperature to test for the chemical stability. CGO-FCO membranes are identified to possess chemical stability in gas mixtures of CO2, SO2 along with oxygen over a period of 200 h at 850 °C under oxygen partial pressure gradient.

La0.98-xSrxCo0.2Fe0.8O3-d (LSCF) is a candidate material for use as an oxygen transport membrane (OTM). In this work, fabrication-relevant properties (sintering behaviour, thermal and chemical expansion) of LSCF (x = 0.2, 0.4, 0.6, 0.8) were investigated in order to select the preferred composition for fabricating a thin-film supported membrane able to withstand the thermochemical stresses encountered during manufacturing and operation with simultaneously high oxygen permeation flux. Partial substitution of La by Sr ions in LSCF is beneficial for increasing the oxygen permeation rate, but it causes drawbacks regarding manufacturing and operation. A Sr content of x ≥ 0.6 results in a swelling of the material during sintering, which complicates the manufacturing of thin, leak-free membranes. This swelling is related to oxygen release during heating, combined with the formation of a liquid phase above 1200 °C. Furthermore, an increase in total strain with Sr content is observed. This is caused by the chemical expansion, while there is no significant change in thermal expansion with increasing Sr content. The compositions x = 0.4 and x = 0.6 showed tolerable expansion coefficients as well as adequate sintering behaviour and were therefore selected for the fabrication of thin supported membranes. These supported membranes with a thickness of 30 µm were manufactured by sequential tape casting and characterised regarding microstructure and oxygen flux.

Porous substrates are a prerequisite for advanced oxygen transport membranes. In particular, phase stability and mechanical robustness are of concern for long term performance and reliability. These aspects were investigated for porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ material using annealing experiments along with microstructural investigations, depth-sensitive micro-indentation and ring-on-ring bi-axial bending tests. Annealing studies revealed phase instabilities at elevated temperatures that were also characterized by X-ray diffraction analysis. Elastic modulus and fracture stress were strongly affected by the porosity, whereas their temperature dependency agreed with the behavior of dense material.

The oxygen transport through supported membranes made of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ is investigated. For this, disc shaped membranes were manufactured by means of tape casting, consisting of a gastight layer with varying thickness (0.9 mm–20 μm) and a porous support with varying porosity (34%/41%). The sample’s microstructure was analyzed using SEM and X-ray computer tomography and by this means characteristic values (i.e., porosity, tortuosity, and specific surface area) were determined. A modeling concept was developed based on literature approaches, extending the Wagner equation for bulk transfer with a geometrical factor β for the characteristic thickness accounting active supports and different surface microstructures. The results were compared with permeation measurements of samples under varying operation conditions (i.e., sweep flow rate, and feed gas). As a result, good agreement between model and measurement in case of a constant porosity is found for characteristic thicknesses L c as reported in literature. However, calculations with varying porosity show indistinguishable results, indicating an underestimate of the geometric factor versus the influence of the characteristic thickness L c . Also, significant limitations of the oxygen permeation due to surface exchange and concentration polarization in the support is shown.

Ceramic mixed ionic-electronic conducting membranes can be used for the separation of single gases (e.g hydrogen or oxygen) from gas mixtures or in membrane reactors. Ceramic membrane reactors (CMR) offer an opportunity to combine separation processes directly with chemical reactions, leading to process intensification and, hence, benefits with regard to efficiency. This integral coupling of both processes, separation and reaction, generates synergies between them. Current research activities focus on membrane reactors due to their high intrinsic efficiency and great potential for the production of a large variety of commodity chemicals, energy carriers, and synthetic fuels. This includes the reduction of CO2 emissions and the utilisation/valorisation of CO2. The development of ceramic membrane materials for such devices is focussed on performance and stability (chemical and thermal) in application relevant environment. The operating temperature is between 400-850°C. The presentation covers a series of potential stable materials for oxygen- and hydrogen ion conducting membranes and their characterisation and performance in application oriented conditions. To reach a high performance of the membrane systems thin film membranes, active surface layers and thermochemical and -mechanical stable supports with designed porosity are required. The production and characterization of membrane structures is explained using the example of sequentially tape cast and laminated supported membranes. Furthermore, the ion and gas transport through such supported thin membranes is described and an example of transport modelling will be given. Finally, the construction of a Proof of Concept membrane reactor for oxygen separation in oxyfuel cement industry or the use in fossil fuelled power plants will be shown. The lecture concludes with an outlook on future development directions.

Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF) is widely known as a promising candidate material for oxygen transport membranes (OTMs). In order to maximize the oxygen permeation through such a membrane, the membrane layer should be as thin as possible, which requires a porous support. Because of the expansion behavior of BSCF, porous supports of the same material were developed to avoid failure due to mismatches in the thermal expansion coefficients. For the purpose of minimizing concentration polarization in the support pores, the microstructure of these support-layers has been optimized. For that reason supports with a porosity of up to 41% were developed. Membrane curvature caused by different shrinkage rates during co-firing could be minimized by the use of corn starch as pore former. By increasing the support porosity from 26% to 41%, the oxygen permeation of a supported 20 μm membrane in an air–argon gradient at 800 °C was increased by 50%. Compared to a disc membrane of 0.9 mm thickness the permeation enhancement is 90%.

The implementation of thin-film technologies in energy-related applications, such as special fuel cells and gas separation membranes for low-emission power plants, is essential in terms of enhancing the functionality, reducing operating temperatures, and increasing lifetime. Introducing thin electrolyte layers into solid oxide fuel cells (SOFCs) decreases the internal cell resistance and thus drastically enhances the power density. This supports the goal of reducing the operation temperature from ~800°C to temperatures below 700°C. As the operation temperature is lowered, the temperature-activated degradation processes are slowed down, and 40,000 h of operation becomes feasible. Reducing the thickness of the gas separation membranes also reduces internal losses, and therefore, the rate-limiting steps within the layer. Thinner functional layers possess higher permeabilities but also involve a risk of more layer defects. This also holds for the fuel cells. Thus, the manufacturing of the supports and the intermediate layers is also very important. The paper gives an overview of the application of thin-film technologies to SOFCs and gas separation membranes and highlights the efforts to date. Examples include SOFC stacks operated stably for in excess of 40,000 h and submicron-sized membranes with high permeability and good separation factors.

The development of solid oxide fuel cells (SOFCs) and gas separation membranes for fossil (fuel?) power plants has previously suffered from cost issues like the manufacturing of the core components including i) the ceramic fuel cell and ii) the ceramic membrane, and from insufficient power density (current density or flow rate) on the stack, module or system level. Forschungszentrum Jülich has been working on SOFC development for 20 years, and on membrane development for 6 years. Both energy-related applications are based on similar materials systems, similar micro-structural features (porous-dense, coarse-fine), comparable application parameters (e.g. high temperature) and are manufactured with similar technologies. In the past the focus laid mostly on basic materials research and proving the functionality of the membranes or fuel cells. Meanwhile, one key topic has been the application of low-cost thin-film high-throughput manufacturing technologies. This includes the fabrication of the supports (mostly tape-casting), the coating with functional layers by ceramics technologies (screen printing, roll coating) and the reduction of sintering steps and temperatures. Additionally special thin-film technologies like sol-gel technique and electron beam evaporation / sputtering have also been applied for functional layers, depending on the functional necessities. The presentation gives an overview regarding the state-of-the-art in SOFC and gas separation membrane development at Forschungszentrum Jülich with an emphasis on the manufacturing technologies, resulting in optimized layer micro-structures and thickness. Additionally it summarizes the electrochemical and permeation data obtained so far.

Tape casting is widely used in industrial scale for production of multilayer ceramic capacitors or substrates for different applications. In 2009, it was successfully introduced as standard shaping technology for 3 (BSCF) are shown. The entire scope from the preparation of the used powders, the different manufacturing steps and their optimization potential up to the final tape-cast product will be discussed. The influence of the use of pore forming agents, heat treatment or other parameters during processing will be described in detail. Finally, the option of sequential tape casting of different materials for graded structures as a future step in shaping technology will be presented for different applications.

Doped ceria and ceria based solid oxide solutions show a unique combination of oxygen ion mobility, electronic conductivity, and high catalytic activity for redox reactions. In this work, the minority conductivity of electrons has been measured directly as a function of the composition of ceria–praseodymia based solid solutions in order to maximize the electronic conductivity without depressing the oxygen ion mobility. The influence of Co as well as the Gd/Pr dopant ratio on the electronic conductivity of ceria–praseodymia pellets was studied for the compositions Ce0.8Gd0.2−xPrxO2−δ (0.05 ≤ x ≤ 0.15) with and without an additional Co content of 0.02 with respect to the formula. The Hebb–Wagner polarization technique was used with ion-blocking microcontacts. In the temperature range 700–800 °C, the presence of high amounts of praseodymium increases the p-type conductivity by a factor of more than 10 for oxygen partial pressures higher than 10−10 bar. Co-doped ceria–gadolinia–praseodymia solid solutions showed a further increase of the electronic conductivities in a partial pressure range where the Co-free materials showed the minimum of the electronic conductivities. It is assumed that the effect of the additional cobalt doping is due to electronic short circuits along the grain boundaries via segregated CoO.

Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF) exhibits high oxygen permeability, which is why it is being discussed for gas separation (oxygen transport membrane, OTM) in zero-emission power plants using oxyfuel technology when the membrane is operated in a clean environment, i.e. no flue gas contact. We investigate the influence of membrane processing on microstructure and oxygen permeation. Pure-phase BSCF powder is synthesized using a modified Pechini method. For comparison, commercially available powder is also used, synthesized by a solid-state reaction. Disk-shaped membranes of various microstructures, i.e. closed porosities and grain sizes, are prepared by uniaxial pressing and sintering of the powders processed in different ways. The powders and membranes are characterized by methods including BET, SEM, XRD, and DSC. The microstructures obtained by different sintering conditions are investigated by SEM and TEM. Sintering at 1150 °C leads to incongruent melting of BSCF indicated by DSC. The liquid phase appears at three-phase boundaries grain–grain–air and consists of nearly pure cobalt oxide with small impurities of barium and strontium detected by TEM/EDX analysis. Oxygen permeation of the membranes is measured in an air/Ar gradient depending on temperature and membrane microstructure. The closed porosity of different processed membranes is varied between 2 and 15% with uniform grain sizes in the range of approx. 10 μm. The average grain size is increased from 10 to 45 μm by increasing the sintering temperature. Neither porosity nor the grain size significantly influences the oxygen permeation rate of 1-mm-thick disks in the investigated parameter range.

Perovskitic oxides are widely investigated as oxygen transport membrane materials for the efficient generation of pure oxygen or the use in membrane reactors. However, most of high performance perovskites suffer from low stability in operation conditions. Therefore, solid solutions of SrTi1−xFexO3−δ (STF) are investigated due to the initial high stability of the strontium titanate host lattice. Self-synthesized powders with substitution of Ti by 0%, 25%, 35%, 50%, 75%, and 100% Fe were studied. Crystal structure, functional properties i.e., diffusion coefficient, surface exchange rates, and oxygen permeation rates as well as membrane fabrication and operation related material properties i.e. sintering behaviour and thermal/chemical expansion were investigated. Substitution of Ti by Fe increases oxygen mobility and, hence, oxygen permeation rates, but reduces stability in operation relevant atmospheres such as Ar/4%H2 or CO2. At the same time thermal/chemical expansion increases. This makes the fabrication of supported thin membranes and their integration into membrane modules more challenging. It turned out that 25–35% Fe substituting Ti seems to be a good compromise between structural and functional properties. Oxygen permeation rates achieved are comparable to that of standard materials such as La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF). At the same time stability is higher and thermal expansion coefficients lower compared to LSCF, which makes STF with limited Fe-content (max. 35%) a promising oxygen transport membrane material.

[EN] One of the most promising materials for oxygen separation amongst ceramic oxygen transport membranes (OTMs) is La0.6Sr0.4Co0.2Fe0.8O3 (-delta) (LSCF) due to its relatively high oxygen permeability combined with high stability. In this work, asymmetric thin-film LSCF membranes supported over a porous LSCF support were manufactured by inverse sequential tape casting. Moreover, surface activation was accomplished by depositing a porous LSCF activation layer in order to promote oxygen evolution reaction, i.e. O2- to O-2 oxidation, in the permeate membrane side. In this case, the porous layer allows the surface area available for oxygen activation to be enlarged. Both the manufacturing of the asymmetric thin-film membranes and the surface activation are described in detail. A thorough study of the oxygen permeation is presented for disk-shaped 30 mu m thick LSCF-supported membranes considering the following operating parameters: temperature (1000-600 degrees C), sweep flow rate (300-750 ml min(-1) argon) and oxygen partial pressure in the feed (0.21-1 atm). High permeation fluxes were achieved, e.g., 11.87 ml min(-1) cm(-2) at 1000 degrees C and 300 ml min(-1) argon sweep when using pure oxygen as feed. A change in the apparent activation energy at about 850 degrees C was related to a reversible structural change in the perovskite symmetry (cubic rhombohedral), as revealed by XRD measurements. Furthermore, the application of an activation layer allowed the permeation process to be improved, especially at low temperatures, i.e. below 800 degrees C. Specifically, an improvement of up to about 300% at 600 degrees C is observed upon application of the activation layer. The activated membrane reached a flux of 13.3 ml min(-1) cm(-2) at 1000 degrees C under an O-2/Ar gradient. Additional permeation tests using CO2-rich sweep gas demonstrated the good stability and performance of these LSCF asymmetric membranes at 900 and 1000 degrees C. (c) 2013 Elsevier B.V. All rights reserved., Financial support by the Spanish Ministry for Science and Innovation (ENE2011-24761 and SEV-2012-0267 Grants), by the EU through FP7 NASA-OTM Project (NMP3-SL-2009-228701), and by the Helmholtz Association of German Research Centres through the Helmholtz Portfolio MEM-BRAIN is gratefully acknowledged. The authors would like to express their gratitude to Dr. W. Fischer for crystal structure analysis by X-ray diffraction. Mrs. J. Carter-Sigglow has contributed to this work with the careful revision of the English usage.

[EN] The objective of the Helmholtz Portfolio "MEM-BRAIN" is the development and integration of cermaic and polymeric gas separation membranes for advanced fossil power plants and other applications like biogas processing or processes in the chemical industry. This will be achieved using membranes with a high permeability and selectivity for either CO2 O2 or H2, for the three CO" capture process routes in power plants, thus enabling CO2 to be captures with high-purity in a reaadly condensable form.

[EN] Oxygen transport membranes made of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) were manufactured by tape casting and co-firing. The disk-shaped membranes consisted of a top gastight layer (70 mu m thick) and a porous substrate (830 mu m thick) with 34% open porosity. The variation of the permeation operation conditions allowed (i) the identification of the different limitations steps in the permeation process, i.e., bulk oxygen ion diffusion, catalytic surface exchange and gas phase diffusion in the membrane compartments and porous substrate, and (ii) the ultimate optimization of the oxygen flux. The variables considered in the systematic permeation study included the inlet gas flow rate of the sweep and air feed, the temperature and the nature of the oxygen feed gas (air or pure oxygen). Moreover, the influence of the deposition of a catalytic activation layer (17 mu m thick) made of BSCF on top of the thin gastight layer was investigated. As a result of this parametric study, unpreceded oxygen flux values were achieved, i.e., a maximum flux of 67.7 ml(STP) min(-1) cm(-2) was obtained at 1000 degrees C using pure oxygen as the feed and argon as the sweep, while a flux of 12.2 ml(STP) min(-1) cm(-2) at 1000 degrees C was obtained when air was used as the feed. (C) 2011 Elsevier BM. All rights reserved., Financial support from the Spanish Ministry for Science and Innovation (Project ENE2008-06302 and FPI Grant JAE-Pre 08-0058), EU through FP7 NASA-OTM Project (NMP3-SL-2009-228701), and the Helmholtz Association of German Research Centres through the Helmholtz Alliance MEM-BRAIN (Initiative and Networking Fund) is kindly acknowledged. Mrs H. Burlet has contributed to this work with the careful revision of the English language.

Materials in the system (Co-)Ce0.8Gd0.2−xPrxO2−δ are often used materials in the catalyst technology due to its oxygen storage capacity and are promising candidates for mixed conductive layer on membranes for oxygen separation in the Oxyfuel process. In both cases the material is exposed temperatures around 800°C and is in contact with aggressive gas species like CO2 and SO2, which may react with the ceramic material leading to a drop in their functional properties. In this work ceramic powder species with x = 0; 0.01; 0.03; 0.05; 0.1 and 0.2 were tested in two gas mixtures containing 99 vol.% CO2 and 1 vol.% SO2 on the one hand and 96.3 vol.% CO2, 3.6 vol.% O2 and 0.1 vol.% SO2 on the other hand. The ceramic powders were treated for 5 hours at 800°C. Before and after the treatment a phase analysis was done by XRD. X-ray amorphous sulphates and the belonging cations were quantified by a wet chemical route. Under CO2 gas the ceramic powders show a good chemical stability. The gassing with SO2 containing test gases causes a chemical expansion of the lattice in dependence of Pr content x due to a release of oxygen. Also small amounts of sulphate were found in the gassed samples. A small content of oxygen in the test gas (3.6 vol.-%) reduces the chemical expansion and the sulphate formation during the test significantly. An additional doping with 2 wt.-% Co inhibits the sulphate formation entirely.